1. Field of the Invention
The present invention relates to a power supply system for a negative impedance discharge load for supplying the electric power to the discharge load such as a discharge tube used in gas laser systems.
2. Description of the Prior Art
A power supply system for discharge tubes for generating glow discharges in, for instance, a CO.sub.2 laser system generally comprises a variable DC voltage source, an inverter, a step-up transformer and a ballast resistor.
As is well known in the art, the discharge tubes have a negative impedance characteristic and in order to convert the negative impedance into the positive impedance characteristic as viewed from the power supply system so as to ensure the stable generation of glow discharges, the ballast resistors are connected in series to the discharge tubes. In order to obtain a high laser beam output, a plurality of discharge tubes which generate glow discharges are connected in parallel while the ballast resistors are connected in series to the respective discharge tubes.
In the case of the AC discharge electrode devices, the above-mentioned inverter is so designed and constructed as to produce a high-frequency output and to apply a high-frequency AC voltage across a discharge electrode. Instead of the above-mentioned ballast resistor, a ballast capacitor is connected. In this case, a resistor having a relatively low value is connected in series to the ballast capacitor.
In the prior art power supply systems of the types described above, in order to maintain stable glow discharges, a ballast impedance must be connected to each discharge load such as a discharge tube, but the impedance drop or power loss across the ballast impedance inevitably results so that a power supply system capable of compensating for such an impedance drop or power loss must be provided. Furthermore, when the characteristics of a plurality of discharge loads or ballast impedances vary one from another, the unbalance among the discharge currents between the discharge loads results. As a result, a small current unbalance thus generated is enhanced due to the negative impedance of the discharge loads so that the increase in current flowing through a specific discharge load causes the unbalance of the voltages across the remaining discharge load and consequently the whole discharge loads cannot maintain stable discharges.
SUMMARY OF THE INVENTION
In view of the above, one of the objects of the present invention is to provide a power supply system for a negative impedance discharge load which can maintain the stable glow discharge across the discharge load without the use of any ballast impedance.
Another object of the present invention is to provide a power supply system for discharge loads which can distribute a balanced current to each of a plurality of discharge loads connected in parallel with each other.
To the above and other ends, the present invention provides a power supply system for a negative impedance discharge load comprising a DC source, an inverter for converting the direct current from the DC source into an alternating current, a current transformer for transforming the alternating current derived from the inverter at a desired level and supplying it to the discharge load and current control means for controlling the output current derived from the DC source at a predetermined level.